linux/lib/dynamic_queue_limits.c
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   1/*
   2 * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
   3 *
   4 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
   5 */
   6#include <linux/module.h>
   7#include <linux/types.h>
   8#include <linux/ctype.h>
   9#include <linux/kernel.h>
  10#include <linux/jiffies.h>
  11#include <linux/dynamic_queue_limits.h>
  12
  13#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
  14#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
  15
  16/* Records completed count and recalculates the queue limit */
  17void dql_completed(struct dql *dql, unsigned int count)
  18{
  19        unsigned int inprogress, prev_inprogress, limit;
  20        unsigned int ovlimit, completed, num_queued;
  21        bool all_prev_completed;
  22
  23        num_queued = ACCESS_ONCE(dql->num_queued);
  24
  25        /* Can't complete more than what's in queue */
  26        BUG_ON(count > num_queued - dql->num_completed);
  27
  28        completed = dql->num_completed + count;
  29        limit = dql->limit;
  30        ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
  31        inprogress = num_queued - completed;
  32        prev_inprogress = dql->prev_num_queued - dql->num_completed;
  33        all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
  34
  35        if ((ovlimit && !inprogress) ||
  36            (dql->prev_ovlimit && all_prev_completed)) {
  37                /*
  38                 * Queue considered starved if:
  39                 *   - The queue was over-limit in the last interval,
  40                 *     and there is no more data in the queue.
  41                 *  OR
  42                 *   - The queue was over-limit in the previous interval and
  43                 *     when enqueuing it was possible that all queued data
  44                 *     had been consumed.  This covers the case when queue
  45                 *     may have becomes starved between completion processing
  46                 *     running and next time enqueue was scheduled.
  47                 *
  48                 *     When queue is starved increase the limit by the amount
  49                 *     of bytes both sent and completed in the last interval,
  50                 *     plus any previous over-limit.
  51                 */
  52                limit += POSDIFF(completed, dql->prev_num_queued) +
  53                     dql->prev_ovlimit;
  54                dql->slack_start_time = jiffies;
  55                dql->lowest_slack = UINT_MAX;
  56        } else if (inprogress && prev_inprogress && !all_prev_completed) {
  57                /*
  58                 * Queue was not starved, check if the limit can be decreased.
  59                 * A decrease is only considered if the queue has been busy in
  60                 * the whole interval (the check above).
  61                 *
  62                 * If there is slack, the amount of execess data queued above
  63                 * the the amount needed to prevent starvation, the queue limit
  64                 * can be decreased.  To avoid hysteresis we consider the
  65                 * minimum amount of slack found over several iterations of the
  66                 * completion routine.
  67                 */
  68                unsigned int slack, slack_last_objs;
  69
  70                /*
  71                 * Slack is the maximum of
  72                 *   - The queue limit plus previous over-limit minus twice
  73                 *     the number of objects completed.  Note that two times
  74                 *     number of completed bytes is a basis for an upper bound
  75                 *     of the limit.
  76                 *   - Portion of objects in the last queuing operation that
  77                 *     was not part of non-zero previous over-limit.  That is
  78                 *     "round down" by non-overlimit portion of the last
  79                 *     queueing operation.
  80                 */
  81                slack = POSDIFF(limit + dql->prev_ovlimit,
  82                    2 * (completed - dql->num_completed));
  83                slack_last_objs = dql->prev_ovlimit ?
  84                    POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
  85
  86                slack = max(slack, slack_last_objs);
  87
  88                if (slack < dql->lowest_slack)
  89                        dql->lowest_slack = slack;
  90
  91                if (time_after(jiffies,
  92                               dql->slack_start_time + dql->slack_hold_time)) {
  93                        limit = POSDIFF(limit, dql->lowest_slack);
  94                        dql->slack_start_time = jiffies;
  95                        dql->lowest_slack = UINT_MAX;
  96                }
  97        }
  98
  99        /* Enforce bounds on limit */
 100        limit = clamp(limit, dql->min_limit, dql->max_limit);
 101
 102        if (limit != dql->limit) {
 103                dql->limit = limit;
 104                ovlimit = 0;
 105        }
 106
 107        dql->adj_limit = limit + completed;
 108        dql->prev_ovlimit = ovlimit;
 109        dql->prev_last_obj_cnt = dql->last_obj_cnt;
 110        dql->num_completed = completed;
 111        dql->prev_num_queued = num_queued;
 112}
 113EXPORT_SYMBOL(dql_completed);
 114
 115void dql_reset(struct dql *dql)
 116{
 117        /* Reset all dynamic values */
 118        dql->limit = 0;
 119        dql->num_queued = 0;
 120        dql->num_completed = 0;
 121        dql->last_obj_cnt = 0;
 122        dql->prev_num_queued = 0;
 123        dql->prev_last_obj_cnt = 0;
 124        dql->prev_ovlimit = 0;
 125        dql->lowest_slack = UINT_MAX;
 126        dql->slack_start_time = jiffies;
 127}
 128EXPORT_SYMBOL(dql_reset);
 129
 130int dql_init(struct dql *dql, unsigned hold_time)
 131{
 132        dql->max_limit = DQL_MAX_LIMIT;
 133        dql->min_limit = 0;
 134        dql->slack_hold_time = hold_time;
 135        dql_reset(dql);
 136        return 0;
 137}
 138EXPORT_SYMBOL(dql_init);
 139
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